1. Field of the Invention
The present invention relates to polymer electrolyte fuel cells including humidifiers with humidifying filters.
2. Description of the Related Art
A polymer electrolyte fuel cell (hereinafter also referred to as “PEFC”) includes a stack of cells and separators. The cells each include a pair of electrodes (anode and cathode) and a proton-conductive solid polymer electrolyte membrane arranged between the pair of electrodes. The separators are configured to separate a fuel gas (anode gas) (reductant), such as the hydrogen, from an oxidant gas (cathode gas), such as the oxygen. The separators are also called as bipolar plates and play a role to guide the fuel gas and the oxidant gas into the anode and the cathode, respectively.
When a separator is made of a metal or carbon having electric conductivity, a fuel cell stack may be configured by stacking plural cells so that the separator is disposed between an anode of one cell and a cathode of an adjacent cell. The resulting stacked body (fuel cell stack) can work by collecting current from both ends of the stack.
Such a fuel cell stack may further include a cooling separator configured to allow a coolant to pass through between cells, so as to eliminate heat generated along with power generation.
Electrolyte membranes for use in polymer electrolyte fuel cells (PEFCs) are configured to allow protons (hydrogen ions) to move therein generally with an assistance of water (moisture) present in the membranes. If electrolyte membranes contain insufficient amounts of water, the movement of proton is prevented and the cell voltage decreases. Accordingly, electrolyte membranes should be protected from excessive drying, in order to generate electric power satisfactorily in the polymer electrolyte fuel cell (PEFC).
As a possible solution to avoid excessive drying of electrolyte membranes, there has been proposed a technique of adding an appropriate amount of water vapor (moisture) to a fuel gas or an oxidant gas, and feeding the humidified gas to polymerelectrolyte fuel cells (e.g., JP-A No. 2005-40675, JP-A No. 2004-206951, and JP Patent No. 3029416).
However, the humidifying technique in related art has following disadvantages. In the humidifier, when a dry gas is humidified by contacting a water-permeable layer which separates a wet gas containing a large amount of water vapor from the dry gas, and the water-permeable layer comprises only one layer, it may be difficult to humidify the dry gas appropriately in the case that the operating conditions of fuel cells vary, such as gas flow rates. For example, if the flow rate of the wet gas is relatively smaller than that of the dry gas, the dry gas may not be sufficiently humidified. If it is relatively large, the dry gas is excessively humidified.